Frequency demodulator



June 6, 1944. w. H. Buss 2,350,369

FREQUENCYv DEMODULATOR Fild Sept. 24, 1941 Pele-a uz-wc: Y

Eigz? INVENTOR. MWAE/V h. BL/J'J' HZ'TOPNE Y Patented June 6, 1944 Warren H. Bliss, Orono, Maine, assig nor' to Radio Corporation of America,'a corporation of Delaware Appli at on ept 24, 1 4 Se al No. $12,089

8 Claims.

This invention relates to frequency demodulators or means for converting frequency modula'- tion into amplitude modulation.

Frequency modulation of a carrier wave is used in radio telegraphy and telephony to a certain extent to avoid static and fading effects. In the transmission of pictures or facsimile telegraphy, the percentage of frequency shift in the modulation process is preferably much greater than that used in other branches of the art and the normal types of filter demodulators are not particularly well suited for picture and facsimile uses. This is because the linearity of the voltage-frequency characteristic'does not extend over a sufiiciently wide range.

It is accordingly an object of this invention to produce a demodulator having a much greater range of linearity than the usual converter.

Another object of the invention is to construct a demodulator of two sections, serially arranged so that the departure from linearity in one of the I sections is compensated by a reverse departure of the other section.

Other objects of the invention will appear in the following description, reference being had to the drawingin which:

Figure 1 is a circuit diagram of the demodulator. V I

Figure 2 is a series of graphs showing the voltage-frequency characteristics of the individual eg qns and of the d ulator as a he Referring to Figure 1, section I consists of a filter of the low-pass type with shunt condensers 2, 3 and 4 and serie inductances 5 and 6.. is given as an example only and diiferen'tnum bers of condensers and inductances'may beflused as desired. Resistance I is incorporated in this filter section to control the curvature of the characteristic of this section. This also produces a low Q circuit, Q being the ratio are R Terminal resistance 8 furnishes the proper output impedance for this filter.

I 511 of section II is connected across resistance I5 through condenser- I6.

To couple the demodulator to the input I!- of the next element in the circuit (not shown) a second high impedance coupling such as tube I8 is used. This tube isshown: as a triode, but it may be a pento'de or' a tube ofany other number of electrodes as desired.' 'Iheinput of tube I8 is connected across condenser I I and the cathode circuit may have a negative bias such'as resistor I 9, for example. 'Thi's resistoralsomay have a by-pass condenser 20. The plate circuit of tube I8 is coupled to the input I! by transformer 2| or in any other desired way. The supply leads 22, Z3 may be used to furnish the plate voltage on both of the tubes I4 andv I8. Negative lead '23, as usual, may be rounded. g

The resistance-capacity section is preferably designed so that the successive condensers 9, 'III', .II and resistors I2, I3 have impedance "values considerably greater than those of the preceding elements. For example, resistor I3 may have ten times the resistance value of resistor I2, con denser II may have one-tenth of'the capacity. of condenser I and the. latter one-tenth of the ca pacity of condenser 9. Y

Thevoltage-frequency characteristic of section II of the demodulator is shown by graph A in Eigure 2 It will be seen" that this. graph is] sub stantially linear in respectlt'o. fi e ency response between the points 24' and 25., bu separat sm linearity beyond point 215. The voltage frquen cy characteristic of section I of the; demodulator is shown by graph Bof Figure 2 This graph concave downwards and "the output of the dezn ule r a a Whol vt llb th -Preh st- Qt th var ous values of graphs A and B. The: curve of the products is shown by graph 0 and sumac: tial linearity extends from point 24, frequency ii, to point 26, frequency is. The shape of the curve B between frequencies f1 and Is on graph B is controlled largely by the value 'of resistance 1.

Any desired frequency range may be obtained by the combination of the two sections. All that is necessary is to use suitable values of resistors,

inductances and capacitances in sectionsI and II. V

The coupling tube I 4 can be eliminated, provided that the input impedance of section II is several times the output load impedance of the low-pass K section I, but I have found that the vacuum tube described is a convenient coupling element for the two sections.

It will be understood how this frequency demodulator operates, but it may be briefly said can be used to operate a recorder or any other. indicator after suitable amplification, The lim- -l.

iters, rectifiers and amplifiers referred to, as well as various other units in aradio orwire circuit,

are not shown, as these pers'e form no part of.

the invention.

While I have shown sections. I and II as the ascaeco of the other, whereby the product of the voltages of said characteristics produces a third frequency-amplitude characteristic that is substantially linear over a wide range while providing a large ratio of response at the extremities of said range.

4. In frequency demodulators, a constant K filter, a resistance-capacity filter, said filters having oppositely curved voltage-frequency characteristics, an electron discharge device having its input terminals connected to the output termiqnals of one of said filters and having an impedance in its cathode return circuit connected to the input terminals of the other of said filters,

. and one of said filters having means to produce 'an over-all sloping and linear voltage-frequency characteristic for the combined filters over a Wide range of frequencies.

input and output units, respectively, this of course may be reversed by use of proper coupling arrangements. 7 q 7 Various modifications may be'made without departing from the spirit of the invention. Having described my invention, what I claim is: 1. In a frequency demodulator, a constant K filter-and a resistance-capacity filter coupled in series, one of said filters having means fordetermining the values to cause the voltage-frequency characteristics to depart from linearity in opposite directions and produce a sloping linear characteristic throughout a predetermined range of frequencies. 7 "2. In frequency demodulators, a low-pass resistance-capacity filter having a non-linear voltage-frequency characteristic between two pre determined frequencies, a low-pass inductancecapacity filter having between said predetermined frequencies a non-linear voltage-frequency characteristic of opposite curvature to that of the first characteristic, means for coupling said filters in'series, and a resistor in series with the inductance of the second-mentioned filter of such value as to produce a' sloping over-all voltagefrequency characteristicof'the combined filters that is substantially linear between said prede termined frequencies. V 3. A selective system forjconverting frequency modulation to amplitude modulation comprisinga first selective network arranged to provide a curvedvoltage-frequency characteristic, a'sec- 'ond selective network arranged to provide a voltage-frequency characteristic having a curvature opposite in sign to that of said first network, "and coupling means between said networks in cas cade; one of said networks having 'means for -controlling the voltages so as to cause the curvature of one of said characteristics to ofis'et that 5. In a frequency demodulator, an inductancecapacity filter having a sloping voltage-frequency characteristic curving in one direction between two predetermined frequencies, aresistance-capacity filter having 'a sloping voltagefrequency characteristic curving in the opposite direction between said frequencies, an electron discharge device having its input terminals connected to the output terminals of one of said filters and having an impedance in its cathode return circuit connected to the input terminals of the other of said filters, said inductance-capacity filter having a resistance of such value as to produce between said predetermined frequencies an over-all sloping and linear voltage-frequency characteristic for the combined filters.

' 6. Frequency demodulation apparatus comprising a tube having an anode, cathode and grid, means for subjecting the gridand cathode to frequency-modulated waves, a cathode return impedance in series with said cathode, a frequency-to-amplitude converting circuit having its input terminals connected across said cathode return impedance, and means for utilizing the output of said frequency-'to amplitude converting circuit.

'7. Apparatus as claimed in claim 6, characterized by the fact that said utilization means includes an electron discharge device having its input electrodes connected to the'output terminals ofsaid frequency-to amplitude converting circuit.

8. Apparatus asclaimed in claim 6, characterized by the provision of an additional frequency-to-amplitude converting circuit having its output terminals connected to the grid and cathode of saidelectron'discharge device, and its input terminals to a, source of frequency varied waves. J I

" WARREN H.-BLISS. 

